The UN Security Council has condemned "in the strongest terms" the terrorist attack in Yemen claimed by a local affiliate of Al-Qaeda.

Following the suicide bombing in the capital Sanaa that killed nearly 100 soldiers, the Security Council called all terrorist acts "criminal and unjustifiable."

Earlier, UN Secretary-General Ban Ki-moon strongly condemned the attack, which left hundreds more wounded, and called for the perpetrators to be held accountable. He also urged all Yemenis to reject violence.

Speaking at the NATO summit in Chicago, U.S. President Barack Obama said the United States is very worried about Al-Qaeda terrorism and extremism in Yemen.

"We are very concerned about al-Qaeda activity and extremist activity in Yemen. A positive development has been a relatively peaceful political transition in Yemen and we participated diplomatically along with Yemen's neighbors in helping to lead to a political transition, but the work is not yet done," Obama said.

Obama said Yemen is drawing foreign terrorists who once might have gone to Pakistan's border area, where insurgents gather to fight NATO forces in Afghanistan.

Obama's counter-terrorism adviser, John Brennan, called Yemen's President Abd-Rabbu Mansour Hadi to convey Washington's condemnation of the attack.

Washington is increasing its military support for Hadi's government and the U.S. military has targeted militants in Yemen with drones.

The strikes have frequently killed civilians and are resented by many in Yemen.

The Al-Qaeda affiliate which claimed responsibility for Monday's attack promised more if a U.S.-backed campaign against militants there did not stop.

One investigator said preliminary findings suggested the bomber was a rogue soldier who had somehow evaded security checks rather than a man in a disguise.